Hard metallic composition and contacts thereof



Patented Dec. 11,1934

PATENT OFFEQE JIABD METALLIC COMPOSITION AND CONTACTS THEREOF George N. Sieger, Indianapolis, Ind., assignor to P. R. Mallory & 00., Incorporated, Indianapolis, Ind, a corporation of Indiana No Drawing. Application December 12, 1931,

Serial-No. 580,729

12 Claims. (on. 200-166) This invention relates to a hard metallic composition, the method of making the same and electrical make-and-break contacts formed there- 'from and, more particularly, to a composition of a hard refractory metal carbide and silver, the

method of forming such a composition and electrical make-and-break contacts formed from such a composition.

General objects of the invention are the provisions of such a hard metallic composition hav-' ing/improved electrical and physical characteristics, the improved method of forming the same and improved electrical make-and-break contacts formed from such composition which have low surface contact resistance, high current carrying capacity and a minimum tendency to stick together during operation.

' More specifically, objects of the invention are the provisions of such a composition of hard metal carbide and silver which has an electrical conductivity higher than that of compositions of tungsten and silver and which is comparable to that of pure silver, a high degree of malleability and physical characteristics which tend to insure original shape and surface condition of elements of electrical apparatuses formed from such composition subjected to various types of forces in operation; and a novel method of forming such composition which preferably includes as onestep thereof the mixing of finely divided particles of a hard metal carbide and silver prior to the formation of bodies therefrom.

A further object of the invention is the provision of such electrical make-ancl-break contacts formed from'a material of a hard refractory metal carbide and silver which have a minimum tendency to oxidize and to transfer metal fromone contact to a cooperating contact, a minimum intemal resistance, low surface contact resistance and a minimum surface resistance rise in operation. a

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each ofthe others, the composition and the article possessing the features, properties, and the relation of constituents which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

The composition of the present invention is adapted-for a variety of uses in electrical fields since it has a high electrical conductivity, a high degree of malleability, is resistive to deformation and deterioration under conditions which are encountered in the operations of many electrical apparatuses and which have been found to have deleterious effects on the compositions, alloys or metals recognized prior to the present invention 5 as representing the best available materials. A hard metal carbide, and, more particularly, tungsten carbide has relatively no malleability, a low degree of electrical conductivity and a high melting point. Silver added to the carbide will provide 10 the desired electrical conductivity and malleability. Such a composition primarily consists of a mass of a hard metal carbide and silver dispersed throughout the mass providing electrical conductive paths therethrough. Silver wets hard metal carbides and thus remains thoroughly bound within the mass of carbide under conditions normally deformative and/or destructive to silver bodies.

The percentage of silver inthis hard metallic composition may vary widely, such as from a few per cent to an amount such that the composition contains a relatively small amount of hard metal carbide, dependent upon the characteristics desired. The malleability and electrical conductivity are increased by an increase in the amount of silver. When high percentages of silver are used the mass of hard metal carbide must be highly porous to provide sufficient voids to receive the silver. If a porous base is formed from the hard metal carbide prior to the incorporation of silver many dimculties are encountered in obtaining a rigid body having the required porosity. These difficulties are overcome by the practice of the improved method described herein whereby at least some of the silver is incorporated with the hard metal carbide prior to the formation of bodies or slugs or the composition.

Prior to the present invention the use of electrical make-and-break contacts in various fields has given'rise to many problems, the chief of which are high surface contact resistance, low current carrying capacity, high contact resistance in comparison with the resistance of the remaining portions of an electrical circuit, tendency of the contacts to fuse and stick together during operation, high rate of oxidation of contact surfaces, high rate of transfer from one contact to another, high internal resistance, high rate of surface resistance rise and many other problems.

In accordance with the present invention, it has been found that generally enhanced characteristics can be imparted to electrical apparatus and Imake-and-break contacts which are designed for use in fields where low surface contact resistance and high current carrying capacity are necessary; where the use of small contacts, the resistance of which in itself must be low in comparison with that of the other portions of the circuit, is desired; where the tendency of cooperating contacts to fuse and stick together must be kept to a minimum; in resistance welding; and, more specifically in relays; in circuit breakers; and, in interrupters for welders, by using in their formation a hard metallic composition comprising, in its preferred form, tungsten carbide and silver.

The ingredients and proportions used in the electrical make-and-break contact composition of the present invention are subject to variations within reasonable limits, that is, for example, the silver may vary between about 20 per cent and about 70 per cent of the contact composition, the balance being tungsten carbide. However, the preferred proportions of the ingredients are as follows:

In accordance with the preferred method of forming the composition of the present invention, the tungsten carbide is formed by any well recognized method, reduced to powdered form, and mixed with finely divided particles of silver. The resultant mixture is pressed into porous slugs which are then sinteredand subjected to fused silver which will run throughout the porous structure, impregnation being abetted by capillary action.

Within the range of percentages of silver given above the double addition method of the addition of silver is preferred since it insures a high percentage of silver in the resultant composition. However, it has been found possible when using relatively low percentages of silver within that range to impregnate the porous slugs of tungsten carbide directly with silver without first reducing the former to a powdered form and mixing therewith a portion of the silver. When, however, high percentages of silver are desired it has been found that the percentage'of voids in the porous slugs of tungsten carbide is such as to allow only a portion of the desired amount of silver to be impregnated therein directly. Thus a certain percentage of the silver is added to the tungsten carbide in the manner described above prior to the impregnation of the porous slugs with silver. The chief factors governing the percentage range of silver for electrical make-and-break contacts are internal resistance and physical hardness. Below 20 per cent of silver, the composition is found to have an undesirably high internal resistance and above '70 percent of silver the composition is found to be too soft for many contact purposes. The preferred composition of the material is given above as comprising 40 per cent tungsten carbide and 60 per cent silver.

Following the preferred method the composition having the preferred proportions of ingredients set forth above is preferably produced by mixing about 55 per cent of tungsten carbide powder with about 45 per cent powdered silver metal. This mixture is preferably prepared by ball'milling the tungsten carbide and silver powder for a considerable period of time, such as,

for example, about 3 hours in a mill containing clean steel balls. After the mixing is completed, the material is preferably paraflined with a solution of paraflin and carbon tetrachloride and thoroughly dried to volatilize the carbon tetraper cent by weight of pure silver preferably in the form of shot being placed adjacent the porous slugs in a suitable furnace and the whole being heated to a temperature of about 1350 C. It is obvious, however, that the invention may be practiced by first sintering slugs of tungsten carbide and silver and then impregnating the resultant porous slugs with the remaining portion of the silver.

For the purposes of the present invention tungsten is the preferred hard refractory metal, although other similar metals of the same group and sub-group of Mendelejeifs periodic table, such as molybdenum, which for some purposes prove satisfactory, are contemplated. The tungsten carbide gives to the composition made in accordance with the present invention a desired rigidity and resistance to deformation not present in electrical make-and-break contacts formed from pure silver. In addition, it provides a retaining base for the silver .constituent which thoroughly wets the former. This base retains the silver in a particularly characteristic dispersed manner such that temperatures which will fuse contacts of pure silver, allowing the latter to stick together and deform during operation, will have no deleterious effect upon electrical make-and-break contacts of the present invention. The silver in the composition insures a. high current carrying capacity and low internal resistance in addition to various other characteristics which may be attributed to this constituent.

Without attributing all of the desirable characteristics of the composition of the present invention to either one or the other of the specific constituents, it is found that the composition, the method of making the same, and electrical make-and-break contacts made therefrom in accordance with the teachings of the present invention attain the objects set forth above in an admirable and efficient manner.

Since certain changes in carrying out the above process and certain modifications in the composition and article which embody the invention may be made without departing from its scope, it is intended that all matter contained in the trative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. An electrical make-and-break contact comprising a composition produced by forming a porous mass of silver and a carbide of one of the above description shall be interpreted as illushard refractory metals molybdenum and tungsten and incorporating in the pores of said porous mass a binding constituent of silver. the silver in said composition constituting from about 20 to 70 per cent thereof.

2. 'An electrical make-and-break contact comprising from about 20 to 70 per cent silver and the balance of a carbide of one of the hard refractory metals molybdenum and tungsten, said contact being formed by impregnating with a mass of molten sflver the pores of a porous mass.

consisting of silver binding together particlesof the refractory metal carbide and having a greater electrical conductivity than a contact formed from silver and the hard refractory metal with the amount of silver in the latter equal to th amount of silver inthe former. 3. An electrical make-and-break contact comprising from about 20 to '10 per cent silver and the balance of tungsten carbide, said contactbeing formed by impregnating with a mass of molten silver the pores of a porous mass consisting of silver binding together particles of tungsten carbide and having a greater electrical conductivity than a contact formed from silver and tungsten with the amount of silver in the latter equal to the amount of silver in the former.

and-break contact composition comprising mixing finely divided particles of tungsten carbide and silver, pressing the mixture to form' a porous slug, sintering the porous slug and impregnating the porous slug with silver.

7. The method of producing electrical makeand-break contact composition comprising mixing finely divided particles of a hard refractory metal carbide and silver, pressing the mixture toform a porous slug, placing a body of silver adiacent the porous slug in a furnace and heating to slu placing bodies of silver adjacent the porous slug in a furnace and heating the whole to a sumcient temperature to cause the porous slug to be sintered and to fuse the bodies of silver to cause the silver to impregnate the porous slug.

9. An electrical make-and-ibreak contact comprising from about 20 to'lO per cent of. silver and the balance of tungsten carbide.

10. An electrical make-and-break contact composed ofsilver and tungsten carbide.

- 11. An electrical make-and-break contact composed of silver and molybdenum carbide.

12. An electrical make-and-break contact composed of a refractory metallic constituent and silver, said refractory metallic constituent comprising carbide of one of the refractory metals molybdenum and tungsten.

GEORGE N. SIEGER. 

